An Acoustic Arms Race

Jamming

As soon as engineers developed effective sonar and radar, others set about thwarting it by interfering with the reception or processing of echoes—a process called jamming. Two organizations have contributed enormously to this electronic warfare: the legendary Lockheed Martin “Skunk Works” and a nonprofit international organization called the Association of Old Crows (the name is a play on the Allied radar equipment and operators in World War II, which were known by the code name Raven). Both groups have developed electronic countermeasures and counter-countermeasures in the military arena. This escalation of ploy and counterploy is reminiscent of the evolutionary arms races that fascinate biologists.

Methods of jamming come in two varieties: passive and active. Passive methods include chaff, materials (such as thin aluminum strips or metalized glass fibers) jettisoned by an aircraft to confuse enemy radar about the aircraft’s precise location and movements. Active methods are electronic signals designed to blind or delude the tracking radar. Noise jammers overwhelm the radar receiver with powerful electronic noise that makes it difficult for the receiver to detect the relatively faint target echo. A repeater jammer provides a copy of the real echo but with inappropriate timing, seducing the receiver into detecting a “phantom” object headed in the wrong direction. Active jamming is a tricky business, because the jammer can inadvertently give the radar receiver a new signal to lock onto.

It might seem unlikely that insects could play similar tricks on echolocating bats, but the coevolutionary arms race of bats and insects has been going on for 65 million years—plenty of time to develop sophisticated measures and countermeasures. Members of my laboratory discovered that Ecuadorian tiger moths in the genus Bertholdia (of the subfamily Arctiinae and family Erebidae) produce a cacophony of clicks when they are targeted by an echolocating bat. The moths intercept the sonar signals of an approaching predator using “bat detectors”—ears tuned to high frequencies—and answer them. The anti-bat sounds are produced by blisters of cuticle called tymbal organs located on either side of the thorax. Each tymbal has 30 or so ridges on it, arranged in a striated band. During activation, underlying muscles deform each ridge in succession, producing a train of clicks. The tymbal produces a second train as it returns elastically to its original shape. The anti-bat clicks are produced at a rate of up to 4,500 clicks per second, meaning that over half of the time that the bat is trying to process echoes, it is also receiving spurious moth-created clicks. This behavior is the hallmark of a sonar jammer. The tymbal organs are a characteristic of Bertholdia and its relatives, and their taxonomic distribution suggests that the organ is an ancient weapon against sonar-wielding bats.

My graduate student Aaron Corcoran determined that Bertholdia’s broadband clicks cause hunting bats to miss their targeted prey both in the laboratory and in the field. How do these jamming sounds work? The logic is the same as that described for radar systems. Some have suggested that if moth clicks are sufficiently similar to returning prey echoes in spectral and temporal characteristics, bats might misperceive them as echoes from objects that do not exist, or “phantom targets.” Second, if clicks are sufficiently numerous and intense, they might mask the presence of echoes, rendering the target invisible. A third mechanism is also possible. Clicks that overlap with or closely precede echoes may diminish a bat’s precision in determining target range. The three jamming hypotheses can be differentiated by what the bat perceives: multiple objects surrounding the moth for the phantom target hypothesis, no target for the masking hypothesis and a blurred target for the ranging interference hypothesis. Corcoran’s best efforts so far indicate that the last hypothesis appears to be the answer. Bats miss jamming moths by a degree predicted by the ranging interference hypothesis.

Corcoran also showed that jamming moths produce their signals only when the bat has “locked on” to them and they are in great danger. The moth determines this threat by sensing a combination of increasing bat cry intensity and a decrease in the interval between bat cries. The threshold for sound production in Bertholdia is closely matched to these parameters and allows the moth to unambiguously determine whether it has been targeted.

The latest known escalation of the bat–moth arms race is the discovery of a stealth bat by Holger Goerlitz, Marc Holderied and their colleages at the University of Bristol. The aerially hawking bat Barbastella barbastellus has lowered the intensity of its echolocation calls by 10 to 100 times. This shift allows them to remain undetected by eared moths until they are very close and the outcome is a fait accompli.

We call the ploy and counterploy of bats and moths a diffuse arms race, with multiple species of bats in an evolutionary battle with multiple species of moths. We are just beginning to understand this arms race, and one can be sure that there will be more surprises to come. For one, the tiger moths comprise approximately 11,000 species worldwide. There are an estimated 200,000 other kinds of moths plying the nighttime sky, and that doesn’t even touch on the beetles, katydids, crickets, mole crickets, flies, lacewings, locusts, nocturnal butterflies and mantids that are also jetting about. Any insect that flies after dark must have a strategy for dealing with nature’s ultimate nocturnal predators— echolocating bats. The race is on.